1. Field of the Invention
The present invention relates to a method and apparatus for playing-back MPEG (Moving Picture Experts Group) data, and more particularly to an MPEG data playback method and apparatus that can randomly access MPEG data stored at a variable bit rate (VBR) and can perform normal playback, reverse playback, fast forward playback, or fast reverse playback from any desired point.
2. Description of the Related Art
With the decreasing prices of MPEG encoders, and with increasing speeds of LANs enabling transmission of large-capacity data, MPEG compression schemes are widely used today. In MPEG1 and MPEG2, which provide methods for compressing moving images for storage, the compression is done on the assumption that the data will be played back in the forward direction along the time axis.
There are two methods of compression: constant bit rate (CBR) compression, in which moving images are compressed at a fixed bit rate, and variable bit rate (VBR) compression, in which, in order to maintain near constant image quality, scenes where compression efficiency is low are compressed at higher bit rates and conversely, scenes where compression efficiency is high are compressed at lower bit rates.
In the MPEG schemes, a unit called a GOP (Group of Pictures) is defined as a point that makes random access possible. A GOP begins with an I frame which can be decoded without referring to a previous or future frame, and the I frame is followed by a sequence of P and B frames which refer to previous and/or future frames. Therefore, an image can be played-back in the forward direction from a GOP point; when playing-back an MPEG moving image from an arbitrary point, the image is played back from a GOP point in the neighborhood of that arbitrary point.
In the case of an MPEG file stored at a CBR, any desired GOP point can be found by simple calculation, because the GOP size is determined by the constant bit rate and the number of frames constituting one GOP. On the other hand, in the case of an MPEG file stored at a VBR, it is not easy to find the desired GOP point because the GOP size varies from one GOP to another.
Therefore, in the case of arbitrary stored data to which index information indicating the location of each GOP point is not appended, such as VBR compressed MPEG data downloaded via a network, the entire data is scanned to identify the location of each GOP point, and the GOP points are stored in the form of an index file or like information. Then, by seeking the desired GOP point by using the thus created information, special playback other than playback in the forward direction from the beginning of the data is performed.
As described above, when performing special playback of arbitrary MPEG stored data and, in particular, VBR compressed MPEG stored data, as an index file or the like has to be created prior to the playback, there has been the problem that it takes much time to start the playback in the case of stored data with a high average bit rate or a large amount of stored data.